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Return Air through the back of the Furnace

I'm going to install a furnace at my cabin (about 700 sq. ft.). One great room, two bed rooms, a full bathroom and a small utility closet. The ceilings are eight feet high, the windows are Anderson low-e glass double pane, and the insulation is up to code as well.

The current furnace is is installed in tight quarters in the utility closet, the return air is currently in the back of the furnace and then goes into the great room only.

The old furnace is adequate and works fine, but it is over 30 years old and is time to replace, along with adding a/c.

I'm going to replace it with a 80% Goodman GMH80603AN and a CAPF1824A6DB case coil and a GSX130241 condensor. The manual says not to have the return air off of the back of the furnace. It can go off of the sides or come in from the bottom. I guess I don't understand why this can't be taken off of the back? This would make an easy installation for me if that could be done.

My only option is to make an enclosed furnace stand (like an air handler stand) elevate the furnace on top of the stand and connect it to my current return air in the back of the closet correct?

I'm going to install a furnace at my cabin (about 700 sq. ft.). One great room, two bed rooms, a full bathroom and a small utility closet. The ceilings are eight feet high, the windows are Anderson low-e glass double pane, and the insulation is up to code as well.

The current furnace is is installed in tight quarters in the utility closet, the return air is currently in the back of the furnace and then goes into the great room only.

The old furnace is adequate and works fine, but it is over 30 years old and is time to replace, along with adding a/c.

I'm going to replace it with a 80% Goodman GMH80603AN and a CAPF1824A6DB case coil and a GSX130241 condensor. The manual says not to have the return air off of the back of the furnace. It can go off of the sides or come in from the bottom. I guess I don't understand why this can't be taken off of the back? This would make an easy installation for me if that could be done.

My only option is to make an enclosed furnace stand (like an air handler stand) elevate the furnace on top of the stand and connect it to my current return air in the back of the closet correct?

To elevate the new furnace will not only be the correct way to do it, but it'll also make for a more quiet installation. The new furnace will move much more air, thus it'll be really noisy. If you have the height, put a filter rack underneath the furnace. The shallow ones a good for 1" or 2" filters.

So the real question without using manual J or D calculations, I looked online and asked other people for correct sizing of my system.

I figured that 60,000BTU at 80% efficiency net output is 48,000BTU. Plus, its a 2 stage so it will usually run on the 1st stage most of the time. I usually don't heat the cabin in the winter, so if I do decide to heat it when I'm there, I figure that it will heat the place faster. My current unit is 50,000BTU and is over 30 years old, so I assume the efficiency is less that 80% and that's not doing a great job on initial heating. I took five hours to heat it from 20 degrees to 70 degrees last winter.

I realize its a little large, but I got one heck of a deal for this system, I couldn't pass it up. It was $ shipped to my house.